Polyimide films have high heat resistance and high electrical insulation properties, and even a polyimide film of thin make provides rigidity, heat resistance or electrical insulation properties necessary for handling. Polyimide films are therefore widely used in the industrial fields of electrical insulation films, heat shielding films, base films for flexible circuit boards, and the like. Polyimide films have also recently come to be used in solar cell substrates and the like.
A polyimide film is manufactured by a process including a casting step for flow-casting and applying a film-shaped coating of a polyimide precursor solution such as polyamic acid, for example, on a support, heating and drying the product, and forming a self-supporting film having self-supporting properties, and a curing step for heat treating the self-supporting film and completing imidization thereof. A surface treatment agent or other organic solvent may also be applied to a surface of the self-supporting film and dried to perform a surface treatment as needed after the casting step, and the curing step may be subsequently performed.
Solvents and other volatile substances are removed from the polyimide film by the heat treatment in the curing step, but complete removal thereof is difficult, and small amounts remain in the final product. Patent Document 1 discloses an aromatic polyimide film in which the volatile content is 0.2 mass % or less.
If a polyimide film is not used in an application that involves heating to a high temperature, the occurrence of blistering, foaming, and other defects of the polyimide film during product manufacturing can be suppressed in a manner in which the volatile content is reduced to about 0.2 mass %, as in Patent Document 1.
However, in the case of solar cells and the like, where the polyimide film is heated to high temperatures of 450° C. or higher during product manufacturing, a volatile content of about 0.2 mass % cannot be considered adequate, and there is a risk of foaming, blistering, and other defects. In solar cells in particular, since the polyimide film is brought to a high temperature after a metal or inorganic layer is formed on both sides of the polyimide film, foaming or blistering readily occurs when even small amounts of volatile components are present in the polyimide film.